What would cause the brakes to feel really soft and almost go to the floor? on 2008 Buick Enclave

On occasion, my brakes will get really soft and almost go to the floor when pushed. It will scare you to death. Then, all of the sudden, they are fine. Just every few weeks it will do this. I thought I was imagining this at first, but now my husband drove it and noticed it, as well.

Do yo know a good mechanic in your zip code! If so, test drive is what it needs to determine the problem! Mechanic will know how to check for a master cylinder bypass problem by lightly and constantly applying brakes. Any warning lights on in dash?